For the manufacture of metallic molding products such as automotive bodies and household electrical appliances, metallic materials such as steel sheets, galvanized steel sheets, etc. are formd to metallic molding articles, coated, and assembled. Coating of such metallic molding parts is generally carried out by a serial process which comprises degreasing, surface conditioning, chemical treatment, electrodeposition coating, etc.
The coating of a metallic molding products (hereinafter referred to briefly as a metallic article or, more briefly, an article) is carried out in a serial line comprising a degreasing stage for degreasing the surface of the article, a surface conditioning stage in which the degreasing agent is eliminated from the article surface, a chemical conversion treatment stage, and an electrodeposition coating stage.
This each process of the coating process generally comprises a combination of dipping, circulating spray, and mist spray systems. The dipping system consists in dipping the article in a treating agent in a dip bath. The dipping system is particularly advantageous for the treatment of an article having an internal "pocket" structure because metal particles deposited in the pocket portion can he effectively eliminated. However, when the article is large, the equipment has to be proportionally large-sized so that not only the initial cost is high but the amount of treating agents are large, thus increasing the burden on work for effluent disposal.
The circulating spray system is a system for cleaning an article by ejecting a large amount of treating solution against the article at the rate of, for example, not less than 3 L, usually about 5 L, per 1 m.sup.2 of the article. This circulating spray system reuses the recycled treating solution and, therefore is advantageous over the dipping system in that, inter alia, the amount of the treating solution and that of the effluent can both be reduced. Although this system insures a sufficient cleaning of the article surface, it has the disadvantage that the internal pocket portion of the article cannot be thoroughly treated.
The mist spray system is a system which comprises ejecting a treating solution against the article at the rate of, for example, not more than 3 L, usually about 2 L, per 1 m.sup.2 of the article. This mist spray system is advantageous in that the required amount of treating solution and the amount of effluent are comparatively small and the size of equipment required is smaller. However, just like the above-mentioned circulating spray system, this system cannot effectively treat the internal pocket structure, although it treats the surface well.
As shown in FIG. 4, the conventional process for coating a metallic molding article utilizing a combination of the above-mentioned systems comprises a step of rinsing the article with warm water using a warm water rinse sprayer 51 and/or a warm water rinse dip bath 52, a degreasing step using a degreasing sprayer 53 and/or a degreasing dip bath 54, and a rinsing step using a water rinse sprayer 55 and/or a water rinse dip bath 56. The article is thence dipped in a surface conditioning bath 57 and a chemical conversion treatment bath 58 in series. The thus-treated article is rinsed with a water rinse sprayer 59 and/or a water rinse dip bath 60, further rinsed with a pure water rinse sprayer 61, and finally electrocoated by dipping in an electrodeposition coating bath 62.
In the above series, the rate of elimination of metal particles is about 35% at the warm water rinse stage, about 65 cumulative % until the degreasing stage, and about 90 cumulative % until the rinse stage just before the electrodeposition coating bath 62. Thus, about 90% of the metal particles are eliminated uutil the electrodeposition coating bath 62 and the remainder or about 10% is carried over into the electrodeposition coating bath 62. The metal particles carried over to the electrodeposition coating stage are comparatively large, sized 80 to 200 .mu.m in diameter, and smaller particles sized less than 80 .mu.m, have been eliminated from the article surface by the treatments such as dipping treatments preceding the electrodeposition coating bath 62.
However, since comparatively large particles sized 80 to 200 .mu.m in diameter are not completely eliminated by the above-mentioned dipping treatments, they remain in the interior of the article and when the article is dipped in the electrodeposition coating bath 62, the solution in which has a relatively high specific gravity, they are dislodged from the article, float on the bath, and are deposited on the surface of the electrostatic coating film to cause film spots. Therefore, in order that the incidence of spot in electrodeposition coating may be precluded, it is necessary to remove metal particles of comparatively large size, namely 80 to 200 .mu.m in diameter, in the course up to the electrodeposition coating bath 62.
Japanese Kokai Publication Hei-6-23332 discloses an apparatus adapted to wash the surface-treated metallic article with a non-pressurized water surge shower and, in addition, bubble air through the cleaning bath solution.
Japanese Kokai Publication Hei-5-339766 discloses a cleaning equipment in which a bubbling device is used to generate microfine air bubbles in the cleaning bath. Japanese Kokai Publication Hei-5-110232 discloses a cleaning method which comprises cleaning a metallic article with air bubbles in the cleaning solution. Japanese Kokai Publication Hei-6-179987 discloses an aeration equipment in which an excess of oxygen is introduced into the cleaning water to clean the surface of a metallic article with microfine gas-phase oxygen.
However, by any of these technologies involving the use of air bubbles formed in the cleaning bath to clean a metallic article, it is difficult to dislodge sufficiently the metal particles deposited in the inner cavity or pocket of the metallic article.
Japanese Kokoku Publication Hei-6-71544 discloses a system for surface treatment and cleaning of a metallic article by means of an ultravibrator.
However, since this technology consists in the mere use of an ultravibrator, neither the surface treatment system nor the cleaning system is sufficiently effective in removing the metal particles deposited in the interior of the metallic article. Moreover, metal particles cannot be removed from the interior of the article at its emergence from the cleaning bath. Thus, metal particles remaining in the interior of the article, particularly particles from 80 to 200 .mu.m in diameter, float up on entry into the electrodeposition coating bath to become copresipitated on the electrodeposition film surface, thus giving rise to film spots.
Aside from the above technologies, a method of controlling the angles of immersion and emergence of the article with respect to a treating bath, such as a cleaning bath, has been proposed. However, with this angle control procedure alone, the metal particles once dislodged from the interior of the article upon immersion are deposited on the interior of the article, so that the particles are hardly removed at emergence of the article from the bath. Therefore, the metal particles floating up upon immersion of the article into the electrodeposition coating bath become coprecipitated on the electrodeposition film.